Semiconductor components, for example photodiodes or phototransistors, can be used for detecting light. What is common to said components is that they have a pn junction around which a space charge zone forms, which zone can be enlarged by means of a correspondingly applied external voltage. Light absorbed by the semiconductor body generates charge carrier pairs which can be separated in the intrinsic or external electric field and be conducted to corresponding external contacts. The electric current thus accumulated at the external contacts represents a measure of the incident light.
WO 2005/036646 A1 discloses e.g. a semiconductor circuit comprising a photodiode having two horizontally running semiconductor junctions.
In particular forward-balanced diodes and transistors exhibit diode leakage currents which are present when voltage is applied and also when light is turned off. Said leakage currents limit the sensitivity of the photodiodes and represent a noise source which cannot be exactly separated from the actual photocurrent.
Dark currents are generated by defects in the semiconductor material of the diode. Defects have energetic states positioned between the valence band and the conduction band. Charge carriers can therefore pass into the conduction band much more easily from such defects at a given temperature. If the defect is situated within the field present or within the space charge zone, the charge carriers or charge carrier pairs thus generated are also fed to the corresponding contacts and produce the abovementioned dark current.
Defects occur in particular at phase boundaries or else at surfaces. Defects can also be produced within the semiconductor body by high-energy implantations.
A high-quality photodiode or phototransistor is permitted to have only a minimal dark current. Therefore, endeavors are made to reduce the number of defects. For this purpose, it has already been proposed to produce the semiconductor junction in the diode by means of an epitaxial method by growing a crystalline layer doped oppositely to the semiconductor body. This makes it possible to reduce the crystal defects at the diode boundary layer.
A further possibility for reducing the dark current consists in using a semiconductor having a larger band gap, in which it is more difficult for charge carriers to cross over to the conduction band even in the dark.
The dark current generated by diffusion can also be reduced by a higher doping of the more lightly doped diode layer since the saturation current Js is proportional to e−EG/nT and inversely proportional to N, where EG is the electronic band gap and N is the dopant concentration.
However, surface and interface defects cannot be improved by means of this measure. These are in particular additional defects at interfaces between silicon and silicon oxide, the latter usually being used for defining the active diode zone and for insulating the latter. Furthermore, defects and impurities can arise during the trench etching, during the etching of nitride or during the production of field oxide.